The present invention relates to DC power supplies, and more particularly to an AC to DC power converter in which the efficiency of the power supply is improved by providing a flyback converter with one semiconductor switch with a lower breakdown voltage than its other semiconductor switch, and by reducing the amount of power processed by the post regulator.
DC power supplies may convert an AC voltage, such as found in utility company power lines, to a DC voltage, and in this role are known as off line switchers (or off line converters). For example, an off line switcher may produce a low voltage regulated output, such as 50 volts DC, from an AC power line voltage, such as 166 to 576 volts RMS.
The power factor is a ratio of real input power (watts) to apparent input power (volt-amperes), and is a measure of power service utilization; it is desirably as close to unity as possible. Traditional methods of improving the power factor are generally not applicable to off line switchers because the current drawn by off line switchers is not sinusoidal. Typically, off line switchers draw the input current in short pulses of high peak value, and the power factor may be increased by chopping the line current at a frequency that is high relative to the input AC frequency. Waveforms of appropriate magnitude and shape may be created by storing energy in the primary windings of a transformer and delivering the energy to the load from the secondary windings, such as by using an electronically controlled flyback converter.
With reference to FIG. 1, a power factor corrected off line switcher 10 may include a rectifier 12 for producing a full wave rectified waveform from an AC input. The rectified waveform is provided to a flyback converter 14 for producing an intermediate DC voltage V1 of a predetermined magnitude (for example, with inputs of 120/240 volts, an intermediate voltage may be 400 volts). The DC voltage is provided to a DC-DC converter 16 that processes all of the output power to produce the desired output voltage V2 (in this example, 50 volts). Control circuitry 18 provides a means for feeding back current and voltage to the flyback converter 14 to enable regulation of the DC output and control of the power factor.
The flyback converter 14 may include a transformer that is operated at a very high frequency relative to the frequency of the AC input. The transformer is operated by means of switches, that may be semiconductor devices, connected to the input and output of the transformer that are turned on and off at the requisite frequency. However, when the rectified AC voltage that varies from zero to a peak rectified AC voltage is applied to the transformer as a series of high frequency pulses, energy stored in the leakage reactance and other parasitic reactances may cause high voltages to appear across the switches during the transition to the off state. When the switches are semiconductor devices, these high voltages may exceed the breakdown voltage of the devices and destroy the switches. Thus, additional protection for the switches must also be provided. For example, with reference to FIG. 2 that illustrates a flyback converter 20 of the prior art, operation of a transformer 22 may be controlled by semiconductor switches 24 and 26 that provide energy for storage in inductor 28. In this example, diode voltage clamps 30 and 32 have been provided to prevent the high voltages from destroying the switches 24 and 26. See, U.S. Pat. No. 5,146,396 issued to Eng, et al. Sep. 8, 1992.
The semiconductor switches used in a flyback converter as in the prior art need to withstand the applied supply voltage plus the additional voltage imposed on the transient sink capacitor 132 plus a safety margin. Using the prior art of FIG. 2 as an example, if the circuit is operated on 480 volts RMS the transistors need to withstand the peak voltage (480 .sqroot.2) 679 volts plus 100 to 200 volts that results from energy being stored in capacitor 132 for a total of 879 volts plus a safety margin. Both semiconductor devices having a breakdown voltage of 879 volts plus a safety factor need to be provided. In many AC to DC converters, especially high frequency converters, the switches are desirably MOSFETs.
The efficiency of semiconductor switches, and especially MOSFETs, improves as the breakdown voltage decreases. As is known, MOSFETs with lower breakdown voltages have a lower on-state resistance, resulting in improved efficiency. Thus, the efficiency of off line switchers may be improved by reducing the voltage capability of the switches in the flyback converter.
The DC-DC converters of prior art power converters process 100% of the power supply's output power. For example, when V1 is 400 volts, the DC-DC converter receives an input of 400 volts and converts it to an output of 50 volts. Processing all of the output power introduces inefficiencies that could not be resolved so long as all of the power was processed.
Accordingly, it is an object of the present invention to provide a novel DC power supply and a novel method of converting power that obviate the problems of the prior art.
It is another object of the present invention to provide a novel DC power supply and method of converting power in which one of the semiconductor switches in the flyback converter has a lower breakdown voltage than the other semiconductor switch.
It is yet another object of the present invention to provide a novel DC power supply and method of converting power in which one switch in the flyback converter has a breakdown voltage less than a peak rectified AC voltage, and the other switch has a breakdown voltage equal to or greater than the peak rectified AC voltage.
It is still another object of the present invention to provide a novel DC power supply and method of converting power in which one switch in the flyback converter is clamped to a voltage set by a constant voltage shunt regulator.
It is a further object of the present invention to provide a novel DC power supply and method of converting power in which one switch in the flyback converter is clamped to a voltage set by a voltage shunt regulator with a clamp voltage that is greater than the product of an output voltage of the transformer times a turns ratio of the transformer and less than the peak rectified AC voltage.
It is yet a further object of the present invention to provide a novel DC power supply and method of converting power in which the post regulator processes a fraction of the converter output power.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of preferred embodiments.